feat(discovery): persist video duration_s instead of discarding it (ADR-028)

ADR-023's filterLowValue already fetches each candidate's duration via the
cheap videos.list quota call to drop Shorts/live, then threw it away — the
videos.duration_s column (migration 001) was never written. Carry it onto the
kept domain.Video and have UpsertVideo persist it, COALESCE-preserving a known
value so an unknown (0) re-upsert never clobbers it (the channel_title backfill
stance, migration 014). This is the enabling change for length-aware burst
selection. No new migration — the column already exists.

internal/domain/domain.go

@@ -77,6 +77,11 @@ type Video struct {
 	URL             string
 	PublishedAt     time.Time
 	SeenAt          time.Time
+	// DurationSeconds is the video length in seconds, when known (fetched by the
+	// ADR-023 videos.list enrichment at discovery). 0 means unknown — the store
+	// preserves a previously-known value rather than overwriting it with 0, and
+	// the onboarding burst (ADR-028) treats unknown as degrade-open (kept).
+	DurationSeconds int
 }
 
 // Transcript is the text of a video (or a record that none was available).